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VCA810: Non-linear issue of VCA810

Expert 4090 points

Replies: 14

Views: 607

Part Number: VCA810

Hi team,

   My customer is using VCA810 to control the sine waveform amplitude, see schematic below, but they find that our device will cause a non-linear issue to the input signal.  See test below, this table shows that the VCA810 will affect the input signal and reduce the Vpeak value. This effect is hard to be calibrated. Do you have any idea about how to avoid this effect? Thanks. 

Input signal

Vpp

Data in ADC3424

Gain=0dB

Data in ADC3424 without VCA810
1V 783 843
0.5V 411 422
0.25V 211 212

  • PRobably slew rate compression, what frequency?

    Michael Steffes

  • In reply to Michael Steffes:

    Hi Michael,

        The max frequency is 30M. But customer is using 1MHz signal will also have such problem. Please help share some suggestion for us to find the root cause. Thanks.

  • In reply to Lin_Charles:

    Well a 1MHz test will not have a slew limit, but a 30MHz is probably too high for this part. Not sure why it is not working at 1Vpp, but you should be looking at the VCA820 type part 

    Michael Steffes

  • In reply to Michael Steffes:

    What do you mean by 30MHz is too high for this device? VCA820 is not p2p with VCA810, it is difficult for customer evaluate our device again. Have we done some test in TI lab? We really need some professional suggestions from team to locate the root cause. Thanks.

  • In reply to Lin_Charles:

    Well, I did all these parts and the VCA810 was a process migration of the earlier VCA610 - I re-positioned the VCA810 at that time as a pulse oriented VCA since it has abysmal HD performance, If you are headed to 30Mhz, you are headed to dissappointment with the VCA810, a quick review of the datasheet shows, 

    All of these curves are starting to roll off at 30Mhz, normally you do not want to operate in the rolloff region, 

    And the distortion is amazingly poor vs The VCA820 series (4 parts). We stopped this plot at 10Mhz out of embarrassment. 

    and then another look at this, if you don't care if you have a highly distorted waveform - sure, but the VCA820 series will be far better - sorry they are not pin compatible. Again, I don't know why you are seeing the 1Mhz 1Vpp compression, could be lots of things,but you have more serious issues at 30Mhz. 

    Michael Steffes

  • In reply to Michael Steffes:

    Thank you for your response here. Could you please help recommend the right device to support below requirement from customer's side,

    1. Max input signal frequency 30MHz;
    2. bipolar signal input;
    3. Gain Adjust Range 40dB;
    4. Max Vpp is 2V;

     Will VCA820 support this case? Or any other device can support this case for good linearity? Thanks.

  • In reply to Lin_Charles:

    Thanks for the detail, '

    When you say good linearity, can you specify a harmonic distortion target - I think this is driving an high pass filter in the LMH6609? That is a 900MHz part for a 30MHz signal? Seems kind of fast. What filter cutoff you are you looking for?

    Michael Steffes

  • In reply to Michael Steffes:

    Also do you want a dB/V gain adjust (useful for AGC loops) or a linear V/V gain adjust ?

    Michael Steffes

  • In reply to Michael Steffes:

    Hi,Michael,

    Thank you for your response.

     Use VCA810 to control the  waveform amplitude,then drive an high pass filter in the LMH6609(filter cutoff is 300kHz), then drive an low pass filter in the LMH6609(filter cutoff is 30MHz).

    When VCA810  is at a fixed gain,I do not want the waveform  was distorted.

    I used 1MHz 1Vpp  sine waveform,but  within  the valid range   I  found  the waveform's  peak  was  not linear.

    Thanks

  • In reply to Michael Steffes:

    Hi Michael,

       Thanks. The output of the VCA part will be sent to bandpass filter (300kHz-30MHz), For the 900MHz LMH6609 is enough for 30MHz signal, let us focus on choosing a VCA part in this application. We do not have a specific target for harmonic distortion, the smaller the better. Thanks.

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